DEVELOPMENT OF MICROSTRUCTURE BASED VISCOPLASTIC MODELS FOR AN ADVANCED DESIGN OF SINGLE CRYSTAL HOT SECTION COMPONENTS

Projektdetails

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CSC - Cost-sharing contracts

Ziel

The results developed include: constitutive models taking into account the heterogeneity of the deformation of two-phase single crystalline superalloys on two levels, advanced experimental testing device for internal pressure testing, improved high-temperature measurement methods for local strain fields.

Two single crystal superalloys, SC16 and MC2, were investigated. Both alloys have shown a heterogenous deformation pattern at low temperatures due to shear band formation on a macro-level and an evolving gamma-gamma' microstructure on a micro-level resulting in high internal stresses and a changing global anisotropy of the material. The models and advanced inelastic analysis methods were developed to describe these material phenomena. Supported was the model development by multiaxial mechanical testing and accompanying microstructural investigations.

This report discusses the design of a laboratory prototype testing machine which is capable of subjecting thin tubular specimens to cyclic torsion and/or to a constant internal pressure at temperatures reaching 1050 C. In addition, due to the 'safe' use of borosilicate glasses as the pressure transmitting medium, it offers new possibilities for local strain measurement using a long distance microscope and image analysis techniques. These developments in experimental techniques have been applied to the testing of the SC alloy MC2 in the temperature range of 850-1050 C.The aim of this proposal is to bridge the gap between the microstructural observations and the current modelling applied to two-phase superalloy single crystals. Modern Nickel-Base superalloys consist of a Y phase (Ni3Al) with Ll2 crystal structure. The classical macroscopic or microscopic approaches fail for this type of material, as they do not take into account (1) heterogeneity on a macroscale, due to shear bands, (2) heterogeneity on a microscale related to the presence of two phases, matrix and precipitates; For instance, the evaluation of resolved shear stress used in the field of material science are note very helpful, as they are computed from the macroscopic stress tensor, which is not really active on the level of the slip systems. The present study will provide the physical basis for modelling the damage evaluation and the lifetime of single crystal blades, and, furthermore, numerical tools for an advanced structural design concept.

The study deals with two new superalloys, one for ground turbines, SC16, tested at 650 C and 950 C, and an aeronautical material, MC2, tested at room temperature and 1050 C. It consists of three main sections. In the first one, preliminary finite element calculations will be performed, in order to have a first idea of the stress heterogeneity and to get information on the needed experimental data, specially under multiaxial loading. The second section corresponds to the experimental investigations, including mechanical tests, mlicrostructural observations, and the development of a new technique for obtaining train maps on the surface of the specimens. This type of result is then a fundamental experimental information for the quantitative evaluation of heterogeneity. In the last part, the numerical models will be applied to the real morphologies of the studied materials, and the various approaches, from the macroscale to the microscdale will be connected together.